Japanese Unexamined Patent Publication (Kokai) No. 2005-097175 discloses, for example, a method for producing propylene oxide, comprising an epoxidation step, a propylene recovery step, and a propylene oxide purification step. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2005-097185 discloses a method for producing propylene oxide in which the reaction is carried out at the concentration of cumene hydroperoxide of 2% by weight or less as the concentration of an organic peroxide in a reaction solution upon the completion of the epoxidation step.
However, it was impossible for these conventional methods to efficiently carry out an epoxidation reaction, sufficiently suppress loss of a valuable component such as propylene or propylene oxide, and to sufficiently save energy in a propylene oxide purification step. Thus, a further improvement has been required.